Dose equivalent in rem is used to quantify a measure of radiation damage for radiation protection purposes and is typically measured by a detector with a neutron sensitivity that is proportional to the dose equivalent in rem, or Sieverts independently of the energy or direction of the radiation field. This neutron detector commonly referred to as a remmeter responds to radiation similarly to how the human body would respond. Some devices, such as scintillators and proportional counters, produce a signal with an energy that varies with the neutron energy. The difficulty with detecting neutrons is that thermal, or low energy, neutrons are easy to detect but carry little dose equivalent, and that fast, or high energy, neutrons are hard to detect, but carry a large dose equivalent.
Generally, fast neutrons are detected by moderating a thermal neutron detector so that the detectors response is proportional to the dose equivalent in rem. Hydrogenous materials are the most common moderators used. The weight of the remmeter depends on the maximum energy that is to be measured and is largely due to the amount of moderating material required. A requirement that at least a 50 percent response be achieved at a maximum energy of 14 MeV requires a moderator weighing approximately 20 pounds. Additionally, heavily moderated thermal neutron detectors over respond at intermediate neutron energies (1 to 500 keV), so the efficiency is reduced by absorbing those neutrons that reach thermal energy after crossing only part of the moderator.
The Navy presently uses a remmeter called the AN/PDR-70, which has a right cylinder of hydrogenous moderator surrounding a proportional counter tube. Though its response varies more with direction than a remmeter with a spherical design its efficiency is higher because a cylinder can hold a larger detector. This device has good response but weighs almost 21 pounds and is too heavy for personnel to use for long periods of time.
What is needed is a lightweight remmeter that provides good response across a useful neutron energy range that is more easily portable by personnel.